1. Technical Field
This invention relates to heterojunction bipolar transistors, and more particularly to heterojunction bipolar transistors having graded base doping.
2. Discussion
Heterojunction bipolar transistors (HBTs) are utilized in many important applications such as log amps for avionics, analog to digital convertors, sample and hold circuits, fast processors for high-speed computation, and many other areas. The relatively high cut off frequency of HBTs make them useful, generally, in high-frequency applications such as those at microwave frequencies.
Much work has been done in further improving the performance of heterojunction bipolar transistors, particularly GaAs/AlGaAs heterojunction bipolar transistors. One area of research is to provide a built-in field in the base of such transistors. It is well known that such a built-in drift field in the base adds a drift component to aid minority-carrier transit across the base resulting in reduced base transit time and higher cut off frequency. A built in field in these transistors can also lead to improved common-emitter current gain .beta. due to a reduction in surface and bulk recombination. This drift field can be produced by grading the dopant concentration across the base. In a non-graded base, minority transport is all diffusive. Early silicon bipolar drift transistors achieved this built in field from graded doping which resulted naturally from the diffusion process used to introduce the dopant into the base region. However, this technique has not been applied to npn heterojunction bipolar transistors because, 1) it is difficult to control the epitaxial growth process with sufficient precision to grade the base doping (particularly at high doping levels), and 2) the prevalent assumption that band-gap narrowing associated with high base doping would render the field produced by the graded doping useless.
Currently, quasi-electric fields in the base of HBTs are usually produced by grading the AlGaAs or InGaAs alloy composition (as opposed to the doping) so that the band-gap is reduced across the base from the emitter to the collector. See H. Kroemer, "Heterostructure Bipolar Transistors: What Should We Build?," J. Vac. Sci. Technol., Vol. B1, pp. 126-130, 1983; and P. M. Enquist, et al, "Heterojunction Bipolar Transistor Using Pseudomorphic GaInAs for the Base," Appl. Phys. Lett., Vol. 49, pp. 179-180, 1986.
Non-uniform base doping has been used to generate a built-in field in HBTs, but the doping gradient was not specified. See S. N. Mohammad, et al, "Suppression of the Emitter Size Effect on the Current-Voltage Characteristics of AlGaAs/GaAs Heterojunction Transistors," Appl. Phys. Lett., Vol 56, pp. 937-939, 1990. In brief, a definite performance improvement could be achieved in heterojunction bipolar transistors in a built-in base field could be developed using well controlled graded base doping.
Thus, it would be desirable to provide a heterojunction bipolar transistor with a built-in base field which would improve base transit time and raise the cut off frequency. Further, it would be desirable to provide a technique for accurately controlling a graded doping profile in a heterojunction bipolar transistor.